Porous, thin films are generally formed by a “one-pot” procedure. The matrix of the film is generated from one or more chemical precursors which react together. The porosity arises from the physical order of the film or is generated by removing a sacrificial templating agent around which the matrix is formed. Once the porous film is formed, a post-synthesis process may be applied to the film, such as patterning (to modify its form) and modifying the outer surface of the film.
Porous materials are of particular interest to the semiconductor industry because of the need for low dielectric constant (k) materials in integrated circuit (IC) devices: When the pores of the material are filled with air, the dielectric constant of the material is reduced with respect to its dense, non-porous state. These IC devices include transistors and other electronic components that are connected to each other by metal lines. Dielectric material fills the void that would otherwise separate the wires and various other IC components. The dielectric constant of this material is ideally kept as low as practical, since a lower dielectric constant corresponds to a reduction in the capacitance of the wires to ground, thereby permitting the IC to run faster (by reducing the RC delay of the interconnect structure) while reducing the power required for signal propagation.
Organosilicate materials are widely used in IC devices as the dielectric material, since they have dielectric constants significantly less than that of dense SiO2 (4.0), with some organosilicates having dielectric constants as low as 2.7. If porosity is introduced into the organosilicate material, the dielectric constant may be 1.5 or less. (See, for example, W. Volksen et al., “Porous Organosilicates for On-Chip Applications: Dielectric Generational Extendibility by the Introduction of Porosity”, Springer Series in Advanced Microelectronics, Vol. 9: Low Dielectric Constant Materials for IC Applications, P. Ho (Ed.), Springer, Berlin, 2001.) Unfortunately, porous materials are less mechanically robust than their non-porous counterparts, so they are more likely to be damaged during chemical mechanical polishing (CMP), for example. Dense organosilicates have Young's moduli in the range of 15-20 GPa (as measured by nanoindentation techniques), whereas Young's moduli below 5 GPa for high porosity organosilicates are not uncommon; these values are to be compared with the Young's modulus of SiO2, which is approximately 80 GPa.